NOCE Optical Instrumentation Use 4 — Questions and Answers
Question 1: When using a lensometer on a high-minus lens with significant spherical aberration, what technique improves measurement accuracy?
- Use the full aperture of the instrument
- Stop down the aperture using the iris diaphragm (Correct answer)
- Increase the illumination to maximum
- Rotate the lens to 45 degrees
Correct answer: Stop down the aperture using the iris diaphragm
Stopping down the aperture (using the iris diaphragm) reduces aberrations and makes the target appear sharper for high-power lenses.
Question 2: If a keratometer measures 41.00 D @ 180 / 43.50 D @ 090, what is the simulated K (SimK) or average K?
- 41.00 D
- 43.50 D
- 42.25 D (Correct answer)
- 2.50 D
Correct answer: 42.25 D
The average K (SimK) is the mean of the two principal meridian readings: (41.00 + 43.50) / 2 = 42.25 D.
Question 3: When fitting a rigid gas-permeable (RGP) lens, a fluorescein pattern that shows pooling centrally and bearing peripherally (steep fit) would require what adjustment to the base curve?
- Flatten the base curve (Correct answer)
- Steepen the base curve
- Increase the lens diameter
- Decrease the optical zone
Correct answer: Flatten the base curve
Central pooling with peripheral bearing indicates the lens is too steep; flattening the base curve improves the fit.
Question 4: The non-contact tonometer (NCT) measures intraocular pressure by detecting what?
- Electrical resistance across the cornea
- The time or force needed to applanate the cornea with an air puff (Correct answer)
- Corneal curvature changes
- Pupil dilation response
Correct answer: The time or force needed to applanate the cornea with an air puff
The NCT uses a calibrated puff of air and detects the time or force required to momentarily flatten (applanate) the cornea, correlating this to IOP.
Question 5: A distometer is used in spectacle dispensing to measure which distance?
- Pupillary distance
- Vertex distance from the back lens surface to the cornea (Correct answer)
- Pantoscopic tilt angle
- Segment height
Correct answer: Vertex distance from the back lens surface to the cornea
A distometer measures the vertex distance — the space between the back surface of the spectacle lens and the front of the cornea.
Question 6: When using a lensometer to measure prism in a lens, the target mires are displaced 2 cm on the reticle. What does this represent?
- 2 prism diopters (Correct answer)
- 2 degrees of prism
- 0.2 prism diopters
- 20 prism diopters
Correct answer: 2 prism diopters
On a standard lensometer reticle, each 1 cm (10 mm) of displacement equals 1 prism diopter, so 2 cm = 2 prism diopters.
Question 7: What property of a spectacle lens can be assessed by using a polariscope or strain viewer?
- Lens power
- Internal stress or lens strain (Correct answer)
- Lens color transmission
- UV protection level
Correct answer: Internal stress or lens strain
A polariscope (strain viewer) reveals internal stress and strain patterns within a lens, which can cause optical distortions.
NOCE Optical Instrumentation Use 4
When using a lensometer on a high-minus lens with significant spherical aberration, what technique improves measurement accuracy?